As the demand for travel to distant destinations has increased, the need for faster and more efficient supersonic aircraft has grown. This need is no longer limited to military applications, as demand for improved civilian supersonic transports has greatly increased. Although past generation supersonic aircraft have been operating for decades, they have always been limited in their efficiency. The efficiency of supersonic aircraft affects their range and operating costs. One approach to increasing efficiency has been to improve the engine operation. This has been achieved in part by using air inlets which are designed to be adjusted in flight to control the airflow and optimize engine performance. Although this approach has resulted in increased operating efficiency, it has come at the cost of increased occurrences of inlet "unstarts" and engine compressor stalls, which risk passenger safety and reduce efficiency and engine output.
When the shock wave is maintained at the throat of the inlet, the inlet is defined as being "started". When the shock is not positioned at the throat, the inlet is "unstarted". During an unstart condition there is a greater loss of pressure over the shock wave, which dramatically lowers the efficiency of the inlet. An unstart may cause excess air to spill out from the front of the inlet and produce significant drag. Further, the change in airflow and pressures can cause the engine's compressor to stall, which greatly reduces the engine's power output. Those aboard a supersonic aircraft which experiences an unstart, experience a sudden jolt as the aircraft lurches backwards when the engine thrust abruptly drops. While such a jolt may be acceptable in military applications, it is definitely unacceptable for civilian transports.
To try to solve this unstart problem, various devices have been developed which attempt to limit the occurrence of unstarts. One approach has been to maintain the location of the shock wave by using a series of sensors placed longitudinally along the inlet to measure pressures within the inlet. These pressure measurements are then compared to a fixed set of pressures, which are indicative of proper placement of the shock wave in the inlet throat. In the event the shock wave is improperly positioned a control system takes corrective action by moving a bypass baffle to a more open or closed position, to move the shock wave forward or aft in the inlet.
Since these prior devices sampled the airflow at the inlet, and since they required time to react to changes in the airflow, a built-in lag unfortunately exists. This lag allows relatively sudden changes in the airflow to cause unstarts. Increasing the response speed of the control system has been attempted to reduce unstarts. However as the response speed is increased the control system becomes subject to unacceptable stability problems. Attempts have also been made to use trends of airflow conditions to predict changes to the conditions, so that the control system begins to "react" before the need for it is certain. However, these predictions have not been sufficiently reliable to adequately anticipate future temperature changes. Another approach to the lag problem has been to add an attitude anticipator system to the control system. The attitude anticipator measures the rate of change of attitude and adjusts the inlet geometry and bypass baffle positioning to avoid unstarts. However, this system was limited to changes in attitude of the aircraft, and as such could not anticipate changes in the airflow itself.
Therefore, a need exists for a device which is able to accurately anticipate changes in airflow conditions so as to avoid unstarts in supersonic aircraft engines. The device must be able to sense changes in the airflow conditions prior to arrival of the changes at the inlet, to allow the control system to reposition the bypass baffle in a timely manner.